1975 Johnson 115 & 135 Compression Difference Between Heads

[dvb3394]

My 1975 Johnson 115 has a significant difference in compression between port and starboard side.
Port is 100 and 115
Starboard is 137 and 137

1. I would like to bring up the compression on the port side and would rather not get into a rebuild. Could I gain compression by putting a 135 head on the port side?

2. How many lbs difference in compression is there between a 115 and 135 head?

3. Could I achieve more compression using a thinner head gasket?

Thanks in advance.

 

[ONERCBOATER]

Re: 1975 Johnson 115 & 135 Compression Difference Between Heads

yes you could gain compression using a thinner headgasket... the real question is WHY IS THERE A DIFFERENCE?
maybe you need to try a decarb and see if they equalize out... that would be easy 1st try and cheap.... next up would be to look at the cylinders closely via borescope or by removing heads.... finally have you had the head off... you could have a minor leak between cylinders perhaps.... just a thought on the last one maybe worth looking into.

Sean

 

[wilde1j]

Re: 1975 Johnson 115 & 135 Compression Difference Between Heads

^^^ Agree. Try to find out what's wrong. Changing heads is clearly NOT the things to do. I would try a decarb 1st, in the hope that the port cylinders may have stuck rings, as suggested above. Then look for either a blown head gasket or cylinder damage.

 

[dvb3394]

Re: 1975 Johnson 115 & 135 Compression Difference Between Heads

About 7 years ago it got hot and blew a head gasket on the starboard side. The side with higher compression. The head was warped on that side and replaced.
Heads are 326504 and 326505.

Ive run it as is for the last 5 of those 7 years.

Now I lost all spark and am servicing it. I figured while I was on the project I'd see about addressing the compression issue.

Even with the wonky compression it has run great.

 

[dvb3394]

Re: 1975 Johnson 115 & 135 Compression Difference Between Heads

Thanks fellas. It would make sense there is a non visible head gasket issue on the port side since the starboard gasket blew. Ill try the decarb and see what I get. Then if no difference Ill pull the port head and see what surprises I can find in there.